The disclosed invention relates to a color image projection system.
Color dispersement and recombination are important aspects of color projection systems. In such projection systems, white light is created from, for example, a halogen lamp or an arc lamp. A prism or other such device is used to disperse the white light into three separate color component beams: a red beam, a green beam, and a blue beam. These beams may be directed to three separate liquid crystal image displays that each include numerous individually addressable display cells. The display cells selectively modulate, i.e., interfere with or reflect, a color component beam thus imparting an image to the color beam. The reflected beams are recombined, and when projected on a screen form a full color image. The image displays are often active matrix image displays with display cells arranged in rows and columns. Typically, multiple image displays, one for each light component beam, are used, adding significantly to color projection system costs.
U.S. Pat. No. 4,943,154, issued Jul. 24, 1990, discloses a prior art projection system. As shown in FIG. 1, this system comprises a light source 50; light transmitter light valves 61, 62 and 63; a light combiner 64; a projection lens 65; a drive circuit 67; an input side convergent lens 68; an input side plane mirror 69; a central convergent lens 70; an output side plane mirror 71; and an output side convergent lens 72. The light source 50 comprises a lamp 51, a condenser lens 52, a concave mirror 53, and a heat absorbing filter 54. The lamp 51 radiates a white light containing the three primary colors of red, green and blue. Rays of the radiant light from the lamp 51 are arranged in approximately parallel rays by the condenser lens 52 and the concave mirror 53. More specifically, the rays of light from the center of a luminous element 55 in the lamp 51 are transmitted in parallel to the optical axis 57 by the condenser lens 52. Infrared rays are eliminated from the light rays passed through the condenser lens 52 by the heat absorbing filter 54. The light rays from the light source 50 are separated into beams of red, green and blue light.
A light separator 90 is used to disperse the light from light source 50 into colored beams of red, green and blue. The separator 90 is shown having two plates 92 and 94. Plate 92 has a low transmissivity for blue light but a high transmissivity for green and red. Plate 94 has a low transmissivity for blue and green light but a high transmissivity for red. Put another way, plate 92 reflects the blue light but transmits green and red. Of the green and red light that remains, plate 94 reflects the green but transmits the red. Thus, the two plates may be used to disperse the light before the light enters light valves 61, 62 and 63. As such, red light enters the red light valve 61. Green light is similarly transmitted to the green light valve 62, and blue light is transmitted to the blue light valve 63.
The light valves 61, 62 and 63 are liquid crystal panels each having matrix electrodes. Such panels generally have a plurality of display cells which change from a more scattering state to a less scattering state upon application of an input such as an electric field, magnetic field, etc. The drive circuit 67 produces electric signals R, G and B according to a video signal Y to control the transmittance of pixels in respective light valves 61, 62 and 63. The modulated light outputs from the light valves 61, 62 and 63 are then combined into a composite flux of light substantially to reproduce a color picture and project, by the projection lens 65, an enlarged image of the picture to a screen 66.